The present invention relates to an improved design of type carrying fingers used in belt printer applications and particularly a design which minimizes the breakage of flexible type fingers during impact printing.
U.S. Pat. No. 3,605,610, filed June 4, 1968 by Earle B. McDowell, Clifford M. Jones and Seymour M. DePuy discloses a printing apparatus utilizing a type carrying belt that carrier a number of flexible fingers each of which has a type face for a particular character on one end thereof. U.S. Pat. No. 3,605,613, filed June 4, 1968 by Seymour M. DePuy and Donald G. Hebert discloses such a type carrying belt and the general design of the flexible type fingers. These type fingers are referably made of metallic material and have a shank provided with means for vertically positioning or locating the fingers within slots on the belt.
It will be appreciated that such type fingers must be designed with certain criteria in mind. For instance, the shank portion of the type finger must exhibit a degree of resilience and be rather accurately dimensioned. The type face portion must be suitable for continuous impacting without damage. Such type fingers when struck by hammers are caused to deflect and impact a record medium. During impact the fingers develop compression and tension stresses both due to deflection in response to the hammer force as well as edgewise bending or deflection due to friction forces developed at the point where the type face impacts the record medium while the belt continues its movement. The finger when exposed to repreated stresses over a long period of time, can develop cracks which lead to breakage and termination of finger life. With the trend to higher printing rates, a need has developed for improving the life of type fingers used in impact printing.
An object of the present invention is to provide improved type fingers capable of longer operation at high speeds.
Another object of this invention is to improve the life of flexible type fingers which are stressed during impact printing.
A further object of this invention is to minimize breakage of flexible fingers used in a belt printer.
A still further object of this invention is to provide fingers which can withstand the continuous stressing during impact printing while being provided with means for movably locating the type fingers in a type finger belt.
A further object of this invention is to minimize breakage which occurs at a shoulder provided on a flexible type finer to enable the finger to be locked in a position in the slots provided in a type finger carrying belt.
In accordance with a preferred embodiment of the invention, there is provided a removable type face carrying finger which can be inserted into a finger slot extending through the width of an elastic belt. The belt when rotated causes the inserted finger to be moved past a hammer where it can be struck to cause impact printing against the record medium. The finger comprises a substantially flat elongated member bearing a type face near one end thereof. The type finger is adapted to be inserted into one of the slots with the plane of its length and width substantially parallel to the plane of the belt. In order to fix the finger with respect to the edge surface of the belt nearest the type face, a shoulder is provided in the member extending orthogonally in the direction of the width of the finger and dimensioned to but the edge surface of the belt. The finger is made of flexible material and adapted to deflect in the direction substantially orthogonal to the plane of its width and length when struck by said hammer. The struck finger develops tension and compression stresses at the shoulder location on the member during impact by the finger against the record medium. To minimize breakage of the finger due to said stresses developed in the finger at said shoulder location, the shoulder is located on the trailing edge of the finger as defined by its movement by the belt past a hammer for impact printing. This corresponds to the region of the finger where the member is under compression.